1. Field of the Invention
The present invention relates to a battery capacity indicator, and in particular, to a device for accurately displaying the remaining capacity and the charging state of a rechargeable battery.
2. Description of the Related Art
A battery charge controller, disclosed in U.S. Pat. No. 5,315,228, issued to Hess, et. al., monitors the voltage, temperature, charge and discharge current of a rechargeable battery, to calculate the battery's charge capacity and charge level. Conventionally, each time the battery is fully discharged, any calculated charge level remaining is divided by two and subtracted from the previously calculated charge capacity. When the battery is fully charged, the charge level is set equal to the charge capacity. During subsequent charge and discharge, the current is converted to a Coulomb count and added or subtracted from the charge level to maintain an accurate charge level indication. Fast charge inefficiency due to temperature is considered by subtracting a temperature proportional factor before the charge level of the battery is updated. The charge level, voltage and temperature are used to determine the optimal fast charge termination point to achieve full charge and prevent temperature abuse and overcharging. A fast charge is applied only if the battery is within proper voltage and temperature ranges. The battery charge controller includes a microcontroller within the same battery pack as the battery, which is powered by the battery when AC power is unavailable. The microcontroller consumes very little power and measures circuit errors to assure data accuracy and time periods of self-discharge, and updates the charge level accordingly. The microcontroller also includes a memory to store the battery charge information and a communication port to provide the charge information.
However, the conventional battery charge controller includes an expensive analog-to-digital (A/D) converter for converting the battery voltage into a digital signal. The microcontroller reads the digital signal and switches the charge mode from the fast charge mode to the trickle charge mode if the read digital signal reaches a specified value. For that purpose, the peripheral elements (e.g., resistor and capacitor) of the A/D converter should be very precise. Although the A/D converter has precise peripheral elements, if the resolution of the A/D converter is very low, it is difficult to accurately measure the battery voltage. Generally, as the resolution of the A/D converter increases higher and higher, the A/D converter becomes more and more expensive. When using an A/D converter having the lower resolution, the battery charger may cause undercharge or overcharge of the rechargeable battery. Furthermore, the device may display an inaccurate remaining capacity indication of the battery, which differs from the actual capacity of the battery.